Working with UDP multicast MPEG-TS streams on Linux

A most common task for all OTT and IPTV video services is, of course, to encode a group of TV channels. And the most common way you will be receiving channels is MPEG-TS streams via multicast UDP. In this article we have explained some popular tips and tricks for working with UDP multicast MPEG-TS streams on Ubuntu and CentOS. We assume you have a linux server or a workstation with a root (superuser) access and a bunch of multicast streams available in your network.
Checking a multicast UDP stream
Suppose you know incoming UDP MPEG-TS multicast host and port:

In this URL is called IP address (or host) and 20000 is port. We will use these later as an example for configuration settings.
Using tcpdump
Let's make sure we are actually receiving a stream using tcpdump. You must be root to run tcpdump.
Execute the following command:

> tcpdump -c 10 dst host and port 20000 and multicast
This command tells to capture 10 multicast UDP packets sent to host and port 20000. If the stream is running, you shall get something like:

tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
15:20:58.818832 IP > UDP, length 1316
15:20:58.824080 IP > UDP, length 1316
15:20:58.829467 IP > UDP, length 1316
15:20:58.835816 IP > UDP, length 1316
15:20:58.841053 IP > UDP, length 1316
15:20:58.846617 IP > UDP, length 1316
15:20:58.852052 IP > UDP, length 1316
15:20:58.857893 IP > UDP, length 1316
15:20:58.863787 IP > UDP, length 1316
15:20:58.869017 IP > UDP, length 1316
10 packets captured
70 packets received by filter
4 packets dropped by kernel
If you don't see output and tcpdump seems to freeze, you are not receiving multicast packets. Press Ctrl-C to abort tcpdump and refer to the next section on firewall settings.
Using netstat
An alternative way to check multicast stream is by using netstat tool. You also need to be root to use it.

> netstat -a -u -n | grep
If multicast stream is running, you will see:

udp        0      0*                          
(line may appear twice or more times)
If previous command emits nothing, you are not receiving multicast packets. Refer to the next section on firewall settings.
Firewall and multicast issues with multicast
Very often multicast streams are blocked by firewall. There are various firewalls on linux distributives, we will cover the most popular: ufw (Uncomplicated FireWall) for Ubuntu and iptables for various distributives.

Be advised that by misconfiguring the firewall you may be unable to access your server via SSH! Disabling firewall at all is highly insecure and not recommended.
ufw on Ubuntu
ufw is a default firewall for Ubuntu linux. To operate ufw you must be root.
Check current status of ufw:

> ufw status
If ufw is active, add rule to enable receiving multicast on specified host and port:

> ufw allow to port 20000 proto udp
You may completely disable ufw by typing:

> ufw disable
This will allow all incoming connections which is insecure and really not recommended. Use it only for testing purposes for a limited period of time.
More info about ufw: UFW on Ubuntu
iptables is a more complicated firewall and it could be found on Ubuntu, CentOS and many more linux systems. To operate iptables you must be root.
To check iptables status run:

> iptables -L
It will show status and list of the firewall rules. If you have a fresh installation of OS, you should see empty rule chains:

Chain INPUT (policy ACCEPT) target     prot opt source               destination

Chain FORWARD (policy ACCEPT) target     prot opt source               destination

Chain OUTPUT (policy ACCEPT) target     prot opt source               destination
To allow an incoming multicast stream you need to add a rule by running the following command:

> iptables -A INPUT -p udp -d --dport 20000 -j ACCEPT
Run iptables -L again and you will see:

ACCEPT     udp  --  anywhere            udp dpt:20000
iptables does not save its status after reboot, so you will need to save newly added rules by typing:

> iptables-save
Getting a list of all multicast streams
Sometimes you don't have list of all multicast streams available. You may get a list of all incoming multicasts quickly by running following simple command:

> sudo tcpdump -n -c 100000 multicast | perl -n -e 'chomp; m/> (\d+.\d+.\d+.\d+).(\d+)/; print "udp://$1:$2\n"' | sort | uniq
It may take some time to capture 100000 packets and produce output — press Ctrl-C if you are tired and reduce amount of packets to capture.
This will produce a sorted list of incoming multicasts like:

To make sure it is an actual MPEG-TS stream and read it parameters refer to the next section.
Checking multicast MPEG-TS with ffprobe
ffprobe is a versatile utility to check various type of media files and streams. It comes in ffmpeg package.
Install ffmpeg (on Ubuntu):

> sudo apt-get install ffmpeg
Install ffmpeg (on CentOS):

> sudo yum install ffmpeg
Use ffprobe to check multicast stream:

> ffprobe udp://
For a valid MPEG-TS stream you will get something like this:

Input #0, mpegts, from 'udp://':
 Duration: N/A, start: 89651.690400, bitrate: 192 kb/s
 Program 10106
     service_name    : AChannel
     service_provider: ChannelProvider
   Stream #0:0[0x19a][6]: Audio: mp2 (\[4\]\[0\]\[0\]\[0\] / 0x0004), 48000 Hz, stereo, s16p, 192 kb/s (clean effects)
   Stream #0:1[0x145]: Video: mpeg2video (Main) (\[2\]\[0\]\[0\]\[0\] / 0x0002), yuv420p(tv), 720x576 [SAR 64:45 DAR 16:9], max. 15000 kb/s, 25 fps, 25 tbr, 90k tbn, 50 tbc
Note lines starting with Stream, they contain information about video and audio data: codecs, bitrate, fps etc.
List of references